Multimode planar spectrographs for wavelength demultiplexing and methods of fabrication

#1. Field of the Invention

#The present invention relates to multimode planar spectrographs and methods for their fabrication, and more particularly to planar spectrographs for demultiplexing coarse wavelength division multiplexed (CWDM) optical signals, and methods for fabrication of such spectrographs employing silicon-based processing.

#2. Description of the Related Art

#The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:

#FIG. 1 is a perspective view of a conventional single-mode planar spectrograph with a dielectric slab waveguide fabricated by depositing and etching a plurality of individual glass layers on a substrate;

#FIG. 2 is a perspective view of a conventional multimode spectrograph where a separate grating block is affixed to a stack of thin glass sheets forming a multimode slab waveguide;

#The present invention provides planar spectrographs capable of demultiplexing optical wavelength signals on multimode optical fibers. The present invention provides silicon processing methods to deep etch aberration-corrected concave gratings to realize minimum spectrograph dimensions. Advantageously, the gratings are integrally formed in a substrate, eliminating alignment issues and promoting less expensive fabrication processes. Both glass and silicon-on-insulator slab waveguide schemes are presented, and embodiments satisfying coarse wavelength division multiplexed (CWDM) requirements for 10 Gigabit Ethernet are illustratively presented for understanding the present invention.

#Considered in preferred embodiments of the present invention are flat-field spectrographs in which the positions I of the input and Di (i=1, 2, . . . ) of the diffracted images are designed to lay along a common line, such as, on a straight edge of a chip or substrate so as to facilitate coupling to an output fiber/detector array.

#Referring now in detail to the figures in which like numerals represent the same or similar elements and initially to FIGS. 3 and 4, a slab waveguide spectrograph 100 is shown for one embodiment of the present invention. Spectrograph 100 includes a substrate 102, preferably a silicon substrate, although other substrate materials may be employed if adequate etching processes exist. Substrate 102 is lithographically processed to pattern a photoresist layer on substrate 102. Then, substrate 102 is anisotropically etched through the photoresist mask to transfer the pattern to substrate 102. This process may be performed with multiple mask materials or multiple layers of resist. A recess 108 is formed in substrate 102 to receive a slab waveguide element 106 therein. Recess 108 has precise dimensional tolerances since lithographic processing provides dimensional precision on the order of sub-micron sizes. This precision is also employed to integrally form a concave diffraction grating 110, for example, an echelette grating on a perimeter of recess 108. Grating 110 may include features which are customized dimensionally and may be formed to include different geometries as a function of position along grating 110. Other features may also be formed in substrate 102 during the same or different etching processes, for example, a socket 107 for a collimating input ball lens 109 and/or an input fiber groove 132.